In a conventional obstacle detecting system (e.g., JP 2005-72771A), a clearance sonar sensor is mounted on a vehicle to transmit an ultrasonic wave as a transmitting wave and receive the ultrasonic wave reflected by an obstacle as a receiving wave.
The obstacle detecting system is configured to mask the reception of the receiving wave while the transmitting wave is being transmitted as shown in FIG. 7, so that the transmitting wave and the receiving wave are separated thereby preventing the erroneous reception of the transmitting wave as the receiving wave. In the masking period, no receiving wave is received and hence no obstacle detection can be attained. As a result, the masking period causes a dead zone, in which no obstacle can be detected, near the vehicle.
It is essential to increase both the directivity and the energy of the transmitting wave for detecting an obstacle existing at a remote position from the vehicle. In this instance, it is also essential to transmit the transmitting wave for a longer time period. The masking period thus inevitably becomes longer and the dead zone becomes wider.